The Email Invalid field in Marketo is used to identify records when the lead is created and the email format is not correct, or when the email is a hard bounce that receives a 5## bounce code signaling that the email is no longer valid i.e. the email address does not exist1.
A hard bounce is an email that has been returned to the sender because the recipient's address is invalid. A hard bounce might occur because the domain name doesn't exist, the recipient is unknown, or there is some other permanent error2.
Marketo automatically marks an email invalid when it receives a 5XX bounce code, or if the email format is not correct when the person is created3. The bounce reasons also populate the Email Invalid Cause field in Marketo, which can be used to further analyze the types of hard bounces unique to your email deliverability instance1.
Therefore, the valid reason for a person's Email Invalid field to be marked as True is when the person was sent an email that was undeliverable due to nonexistent email address, which is option B.
References: 1: What is the Email Invalid Field in Marketo and what should you do with it? - Sponge 4: Map the Email Invalid field in Marketo to a custom field in Salesforce? - Marketing Nation Product Discussions 3: Contact Marked Email Invalid - No Record in Activity Log - Marketing Nation Product Discussions 2: Hard and Soft Bounces in Email | Adobe Marketo Engage